Variable displacement pumps require various types of displacement controls dependent upon the usage of the pump. High pressure feedback is almost always a requirement for any variable displacement control. For hydromechanical controls, this feedback is used to actuate a spring loaded spool in either a horsepower or torque limiting control, a high pressure cutoff control, or a load sensing control. Quite often, these controls utilize multiple forms of feedback to perform multiple control functions. When this is the case, the pump controls become very complicated in their design and are often characterized by large hardware configurations. Such large pump controls generally add to the overall cost of the pump.
Moreover, the torque limiting control commonly includes a torque control valve movable to an operating position for causing the swashplate to move toward a minimum displacement position when the discharge pressure exceeds a predetermined high pressure level. The torque control valve is moved toward the operating position by a feedback piston disposed within an actuating chamber having pump discharge pressure directed thereto. One or more springs are disposed to resist movement of the torque control valve to the operating position to establish the torque level setting. A problem encountered therewith is that when the power source driving the pump is shut down, the spring or springs bias the torque control valve to a position causing the swashplate to move to its maximum displacement position. This adds a considerable amount of extra torque to the power source that must be overcome by the starter to get the power source up to start-up speed when the power source is being started.
Finally, the maximum and minimum displacement settings of the swashplate are often established by contact between the swashplate and mechanical stops. This creates a hard stop which generates excessive noise.
The present invention is directed to overcoming one or more of the problems as set forth above.